The invention is concerned with electro-explosive devices or "initiators", and it has particular application to initiators which are known to the art as "squibs". The invention is directed to an improved squib or initiator which is constructed to be immune from premature firing by static electricity.
As discussed in U.S. Pat. No. 2,802,421, which issued Aug. 13, 1957, the art has long recognized the dangers in the accidental discharge of electric initiators by static electricity. Accidents, which at the time of their occurrence seemed without explanation, have been subsequently traced to the firing of an initiator by a static electricity discharge. Since conventional ignition compositions necessarily are highly heat sensitive, a discharge of relatively high voltage from static electricity is quite capable of igniting the composition and firing the initiator. The art has generally considered that accidental firings result from a direct static discharge from a lead wire to the metallic case of an initiator in the locus of the ignition composition.
The danger of premature firing due to static discharge is present in all types of electric initiators. A structure has previously been proposed in the prior art in which a lead wire is disposed in contact or almost in contact with the metallic case of the initiator, at a point removed from the ignition composition, in order to provide for a discharge of the static electricity from the lead wire to the case. However, this type of structure offers good protection from static or firing of the initiator only when the charge passes down the one lead wire to the case. Little or no benefit is obtained when the current passes through both lead wires. Even when both lead wires are so disposed, discharge will occur from only one wire in many instances, and protection will be limited as far as heating of the bridge wire is concerned.
A structure has also been proposed in the prior art wherein one or both of the lead wires are connected to the metallic case of the initiator by means of semiconductive material outside the locus of the ignition composition. This construction affords a considerable improvement in resistance to static discharge. However, it is very difficult with such a structure to maintain a proper balance of conductivity that will allow a discharge from both wires to the case and still have sufficient resistance for protection against the low voltage currents which attend many commercial operations. Additionally it has been found that in some instances, the static resistance of this type of structure diminishes with storage. Furthermore, such initiators have been known to fire when 10-40 volts from a battery are applied between the lead wires and shell.
In another proposed structure, conductive material is disposed about the bared lead wires and extends to the case. This conductive material acts as a true resistor in that the resistance is low, normally form 10-100 ohms. The resistance of such a body of material is similar to that of a regular carbon resistor, being fairly constant, but subject to variation due to temperature. The resistance does not change greatly due to passage of current until the current is sufficient to cause heating. This type of initiator gives good static protection in most instances. However, it has been found that in some instances discharge occurs from only one wire which allows a firing of the initiator by the heating of the bridge wire. Even more than in the case of semiconductive material, this structure has a serious deficiency of insulation from case to lead wires and can be fired in this manner with a very low voltage. In other words, this structure, while removing a considerable part of the hazard of static electricity, has introduced an equally undesirable hazard in the form of undesirably low resistance between lead wires and case.
In still another structure, it has been proposed to equipt the bared lead wires with protrusions which extend toward the metallic case. This structure is usually employed with a matchhead ignition element which is insulated. However, a discharge usually takes place from only one wire and a firing of the initiator by the heating of the bridge wire is thus permitted. In addition, a hotter spark is obtained when the discharge is directed from localized points. Even though the protrusions are outside the locus of the ignition composition, such violent discharges are to be avoided when the same results can be obtained in a less violent manner.
While all of the initiators described exhibit a measure of resistance to static electricity discharges, it will be seen that in each case, the protection from static electricity is not complete and, in most instances, what protection is obtained is brought about by a structure which is characterized by low voltage breakdown.
Ignitors have found widespread use as airbag inflators in motor vehicles. The initiator commonly used in such applications is a two-pin squib with a "floating" electrically conductive case. In such an initiator, the firing signal is applied by way of an electric circuit located within the case, but insulated therefrom, the electric circuit being connected to the pins of the squib to energize a bridgewire within the squib. In the two-pin squib, the bridgewire is connected between the two-pins rather than between one pin and the case, as is the case in the coaxial type of squib.
Specific attempts have been made in the prior art to render the "floating" case initiator described in the preceding paragraph immune from electrostatic discharges. These attempts have included the use of a "leaky" insulator. As described in U.S. Pat. No. 3,783,788, electrostatic charges accumulating between the pins, or between either pin and the case, are dissipated by a low-level current flow through the insulator. However, this approach has met with but limited success. Another attempt in the prior art to render such initiators immune from electrostatic discharges has involved the use of Zener diodes connected between the pins and between each pin and the case. This arrangement also is met with only limited success, and has proven to be cost prohibitive.
U.S. Pat. No. 4,261,263, assigned to the present assignee, describes and claims yet another approach to immunity from radio frequency signals and electrostatic discharges, which has proven to be highly successful. The initiator described in the discharge of radio frequency and electrostatic energy, the spark gap being isolated from the pyrotechnic charge within the initiator.
An objective of the present invention is to provide an improved and simple initiator which may be of the "floating" case. type or of the coaxial type, and which is constructed to exhibit high immunity to electrostatic discharges resulting from ambient electrostatic fields.
The electrostatic discharge test specifications of m automobile manufacturing companies requires that a 500 pf capacitor charged to 25 kilvolts be discharged through the initiator by way of a 5 kilo-ohm series resistor. In the case of the floating casing type of initiator, the test is specified to be conducted in three formats, namely pin-to-pin, each pin-to-case, and the short-circuited pins-to-case. The electrostatic discharge test specification for one major automobile manufacturer requires that a 150 pf capacitor charged to 25 kilvolts be discharged through a 150 ohm series resistor.
The latter test dissipates approximately 10 times the energy in the initiator as compared with the former test, and this has resulted in a high number of firings in the lead-to-lead 1 test mode when the prior art initiators were tested. This is because the energy dissipated in the bridgewire is comparable with the energy required to fire the initiator. For that reason, the prior art initiators for the most part have been found to fail the severe test.